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1、Source:MaterialsHandbookPart1Materials,TheirPropertiesandUsesDownloadedfromDigitalEngineeringLibrary@McGraw-Hill(www.digitalengineeringlibrary.com)Copyright©2004TheMcGraw-HillCompanies.Allrightsreserved.AnyuseissubjecttotheTermsofUseasgivenatthewebsite.Materials,T
2、heirPropertiesandUses2ABLATIVESABLATIVES.Materialsusedfortheoutwarddissipationofextreme-lyhighheatsbymassremoval.Theirmostcommonuseisasanexternalheatshieldtoprotectsupersonicaerospacevehiclesfromanexcessivebuildupofheatcausedbyairfrictionatthesurface.Theablativema
3、terialmusthavealowthermalconductivityinorderthattheheatmayremainconcentratedinthethinsurfacelayer.Asthesurfaceoftheablatormeltsorsublimes,itiswipedawaybythefric-tionalforcesthatsimultaneouslyheatnewlyexposedsurfaces.Theheatiscarriedoffwiththematerialremoved.Theles
4、smaterialthatislost,themoreefficientistheablativematerial.Theablativemater-ialalsoshouldhaveahighthermalcapacityinthesolid,liquid,andgaseousstates;ahighheatoffusionandevaporation;andahighheatofdissociationofitsvapors.Theablativeagent,orablator,isusuallyacarbonaceo
5、usorganiccompound,suchasaphenolicplastic.Asthedissociationproductsarelostasliquidorvapor,thecharisheldinplacebytherefractoryreinforcingfibers,stillgivingameasureofheatresistance.Theeffectivelifeofanablativeisshort,calculatedinsecondspermillimeterofthicknessforthed
6、istancetraveledintheatmosphere.Singleablativematerialsseldomhaveallthedesirablefactors,andthuscompositesareused.Phenolicorepoxyresinsarereinforcedwithasbestosfabric,carbonizedcloth,orrefractoryfibers,suchasasbestos,fusedsilica,andglass.Therefractoryfibersnotonlyar
7、eincorporatedformechanicalstrength,butalsohaveafunctionintheablativeprocess,andsurface-activeagentsmaybeaddedtospeedtherateofevaporation.Anothercomposite,polyarylacetylene(PAA)reinforcedwithcarbonfiberfabric,provedsuperiortocarbon-reinforcedphenolicinteststodevelo
8、panalternativeablativeandinsulativematerialfornozzlecomponentsofsolidrocketmotors.FavoringthePAAisitshigh(90%)charyield,lowerweightlossanderosion,greate
